1. Field of the Invention
The present invention relates to an apparatus for forming glass, more particularly an apparatus for forming a glass sheet.
2. Technical Background
The manufacture of sheet glass by downwardly flowing molten glass over a forming wedge is well known in the art. However, in practice it was found that the actual width of usable sheet produced was substantially less than the maximum width of usable glass sheet which theoretically could be obtained, as predicated by the longitudinal length of the forming wedge. That is, it has been found that when the molten glass flows along the underside of a negatively inclined surface, such as the downwardly converging surfaces of a forming wedge, the width of the glass stream contracts and pulls inwardly from the vertical ends of the wedge.
This lateral movement of the molten glass produces a bead or thickening portion along the edge of the sheet—the edge head.
The thickened sections on the edges of the sheet not only decrease the amount of usable uniform thickness sheet glass that can be obtained from a given width of drawn sheet, but also limit the speed at which the sheet may be drawn. That is, since the sheet is annealed as a continuous process immediately after formation, and the time required for sheet glass to reach an acceptable strain level in passing through the annealing portion of the process is directly proportional to the thickness of the glass, sheet having thickened edge portions requires more annealing time, thus limiting the maximum rate of sheet formation.
In the past it has been suggested that the narrowing of the width of glass sheet formed by the downdraw process may be prevented by utilizing rollers or edge cooling.
However, this has not been entirely satisfactory since rolling merely attempts to reduce the thickness of the beaded portion without attempting to correct the cause or source of the bead, and cooling tends to enlarge the formation of beaded edges, which as previously indicated are detrimental in the formation of sheet glass. Further, high viscosities adjacent the edges of the sheet flow actually tend to create a pull-in or narrowing of the glass to a much greater extent than do low viscosities. However, should the edges be chilled sufficiently to set up the glass, additional detrimental effects are produced in the form of sheet warpage and undesirable stresses.
Prior art methods of increasing sheet width have employed a web surface portion extending between the downwardly converging surfaces of the forming wedge and a projecting edge surface portion which aided in increasing the width of the drawn glass sheet. U.S. Pat. No. 3,451,798 discloses such a web surface portion which terminates at its lowest extent at the horizontal plane passing through the root, the line along which the downwardly converging surfaces of the forming wedge meet. U.S. Pat. No. 3,537,834 discloses a forming apparatus comprising a web surface portion which, at its lowest point may be extended below the root.
In one prior art apparatus, illustrated in FIG. 1, an overflow trough 10 comprising converging forming surfaces includes multiple edge directors 12. Each edge director 12 comprises two main portions: a projecting edge surface portion 14 which intersects forming surface portions of the trough along its vertical extent, and a web or filleted surface portion 16 which extends between the projecting edge surface portion 14 and one of the downwardly inclined converging surface portions. However, web surface portion 16 does not extend below the lower apex formed by the converging forming surfaces.
In another prior art apparatus, shown in FIG. 2, web surface portion 20 extends below the lower apex, or root, as indicated by point 22. However, whether web surface portion 20 is planar, curvilinear or frusto-conical in shape, the extended web surface portion does not break or kink (i.e. demonstrate an abrupt change in direction) along its downward length.
Although the prior art devices described above have been useful to extend the width of glass sheet drawn from overflow troughs, devices capable of still further improvement in sheet width are needed. Unfortunately, there are practical limits to the length of the forming wedge without incurring problems associated with sagging of the wedge. Thus, there is a need to further increase the width of glass sheet drawn from a forming wedge which does not entail increasing the length of the forming wedge itself.